This invention relates to magnetic resonance imaging (MRI) methods and systems and, more particularly, to the imaging of moving three-dimensional subjects such as coronary arteries.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field along the z-axis of a Cartesian coordinate system and consequently precess about the polarizing field in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to an excitation magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment M.sub.z may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation field B.sub.1 is terminated, and may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR (nuclear magnetic resonance) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The imaging of three-dimensional subjects can be accomplished using either two-dimensional (2D) or three-dimensional (3D) MR imaging methods. The acquisition of a 3D image requires the repeated performance of a 3D imaging pulse sequence during which two separate phase encoding gradients are stepped through a large number of values. To acquire a high resolution 3D image, more phase encoding steps are required and this causes the scan to become too lengthy for many clinical applications.
A high resolution 2D MR image can be acquired in a much shorter scan time. To examine a three-dimensional subject therefore, it is common practice to acquire one or more 2D slice images that have been precisely located with respect to the subject. Precise location may be accomplished by acquiring a low resolution MR image and using an interactive display to graphically prescribe the precise location of a subsequent high resolution 2D scan. This strategy can be used successfully when the subject is stationary and the region of interest can be located in a 2D slice image.
In many clinical applications, high resolution images of moving 3D subjects must be acquired. One such application is imaging of the coronary arteries, three-dimensional objects that move substantially during the cardiac cycle. It is sometimes possible to position a 2D "slice" acquisition to capture a portion of the subject coronary artery in the 2D imaging plane at some point in the cardiac cycle, but it may not be possible to capture the same portion at other cardiac phases because the vessels move in and out of the 2D slice constantly as the heart beats. In addition, imaging all of the coronary artery segments of interest may require many separate 2D image acquisitions.